Topic
Very low frequency
About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.
Papers published on a yearly basis
Papers
More filters
15 Apr 2021
TL;DR: In this article, the authors proposed using many short boxcar gates, denoted sub-gates, and combining the subgates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR.
Abstract: . The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures, but data are inevitably contaminated by noise from various sources. It is common practice to gate signals from TEM systems to reduce the amount of data and improve the signal-to-noise ratio (SNR). Gating acts as a filter, and optimum gating will pass the TEM signal un-attenuated while suppressing noise. In systems based on analog boxcar integrators, the gating corresponds to filtering with a square window. The frequency response of this window shape has large side lobes, which are often insufficient in attenuating noise, e.g., from radio signals in the very low frequency (VLF) 3–30 kHz band. Tapered gates have better side lobe suppression and attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using many short boxcar gates, denoted sub-gates, and combine the sub-gates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR. The semi-tapering approach is analyzed and tested experimentally on data from a roving TEM system. We quantify the effect of semi-tapered gates by computing an improvement factor as the ratio between the standard error of data measured with boxcar gates and the standard error of data measured with semi-tapered gates. Data from a test survey in Gedved, Denmark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 µs . After inversion of the data, we find that semi-tapering increases the depth of investigation by about 20 % for this specific survey. We conclude that the semi-tapered approach is a viable path towards increasing SNR in TEM systems based on analog boxcar integrators.
4 citations
11 Dec 2012
TL;DR: In this paper, the authors presented the nighttime D-region ionosphere characteristics of tweeks at Gakona station (6271° N, 14399°W) in the North America Region by using the fundamental of cut-off frequency.
Abstract: Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics generate from lightning discharges and spread in the Earth-ionosphere waveguide mode This study uses the electromagnetic wave approach describe as ELF (extremely low frequency) and VLF (very low frequency) which propagates sub-ionospherically among the earth and the lowest ionosphere This paper presents the nighttime D-region ionosphere characteristics of tweeks at Gakona station (6271° N, 14399°W) in the North America Region By using the fundamental of cut-off frequency, we estimated the nighttime variations of the tweek reflection height of the D-region ionosphere, equivalent electron density and the distance travel by tweek along its propagation Our observation indicated that the equivalent electron densities for tweeks varies from 23–27 el/cm3 in the altitude range of 77 to 91 km and demonstrated that these signals is capable to travel up to 5600 km over a wide area in mid-latitude region Comparison in term of the ionospheric parameters at different latitude shows that the reflection height and electron density in this study is almost consistent for all latitude tweek measurements
4 citations
TL;DR: In this paper, phase and amplitude perturbations on VLF subionospheric transmissions from transmitter NWC to Dunedin have been studied on both MSK frequencies and at spaced receivers, 9 km apart.
4 citations
TL;DR: In this article, the effect of the initial pump wave on the generation of whistlers by a weakly unstable background distribution was studied. But the model was not applied to the case of a very low-frequency (VLF) signal.
4 citations
TL;DR: Instrumentation, data processing and preliminary results of the EXOS-B VLF wave receiver are presented in this article, which is connected, by command, either to a long dipole antenna or to a loop antenna, and observes both electromagnetic and electrostatic waves in a frequency range between 150Hz and 95kHz.
Abstract: Instrumentation, data processing and preliminary results of EXOS-B VLF wave receiver are presented The VLF receiver is connected, by command, either to a long dipole antenna or to a loop antenna, and observes both electromagnetic and electrostatic waves in a frequency range between 150Hz and 95kHz Up to the present time after the launch, various VLF phenomena including whistler echo trains, whistler triggered emissions, hiss, chorus, and possible power line harmonic radiation have been observed In addition to these natural phenomena, artificial VLF waves excited by electron beam injection have also been observed
4 citations